Gov. Ron DeSantis of Florida officially entered the presidential race last week, but he appears farther than ever from the one-on-one matchup that his allies believe he needs to wrest the nomination from former President Donald J. Trump.

Former Vice President Mike Pence is burrowing deeper into Iowa, crucial to his effort to dislodge the Republican front-runners, even before he has announced his bid. Former Gov. Chris Christie of New Jersey is intensifying preparations for another campaign, with an expected focus on New Hampshire. And Republican donors and leadership on Capitol Hill are showing fresh interest in Senator Tim Scott of South Carolina, who kicked off his campaign last week. Even candidates who have barely been mentioned are suddenly expressing interest in 2024.

The rapidly ballooning field, combined with Mr. Trump’s seemingly unbreakable core of support, represents a grave threat to Mr. DeSantis, imperiling his ability to consolidate the non-Trump vote, and could mirror the dynamics that powered Mr. Trump’s takeover of the party in 2016.

It’s a matter of math: Each new entrant threatens to steal a small piece of Mr. DeSantis’s potential coalition — whether it be Mr. Pence with Iowa evangelicals or Mr. Scott with college-educated suburbanites. And these new candidates are unlikely to eat into Mr. Trump’s votes. The former president’s base — more than 30 percent of Republicans — remains strongly devoted to him.

“President Trump — he should go to the casino, he’s a lucky guy,” Dave Carney, a veteran Republican strategist based in New Hampshire, said of the former casino owner, Mr. Trump.

“It’s a gigantic problem” for Mr. DeSantis, added Mr. Carney, who has worked on past presidential campaigns, because “whatever percentage they get makes it difficult for the second-place guy to win because there’s just not the available vote.”

Mr. Trump’s advisers have almost gleefully greeted each successive entry as part of a divide-and-conquer strategy that his team has spoken about since 2021. And many of the candidates seem more comfortable throwing punches at Mr. DeSantis than at Mr. Trump.

The DeSantis campaign sees the landscape differently.

“We don’t believe it’s 2016 again,” Ryan Tyson, a senior adviser to Mr. DeSantis, said in an interview.

And in a private briefing for donors this week, Mr. Tyson described a Republican electorate split into three parts: 35 percent as “only Trump” voters, 20 percent as “never Trump” and the remaining 45 percent as the DeSantis sweet spot.

Mr. Tyson told donors, in audio that was leaked and published online, that every entrant besides the two front-runners were isolated in the “never Trump” segment. “If your name is not Ron DeSantis or Donald Trump, you are splitting up this share of the electorate,” he said.

In the months leading up to his campaign launch, Mr. DeSantis and his allies framed the 2024 primaries as a two-man race. But as he has stumbled in recent months, amid questions about his personality and political dexterity, rivals have become emboldened. And some have the cash to stay relevant deep into the primary calendar.

Mr. Scott entered the race with nearly $22 million on hand, and he raised $2 million more in his first day as a candidate. The wealthy, little-known governor of North Dakota, Doug Burgum, now sees a 2024 opening, filming ads recently to prepare for an imminent campaign, according to two people involved in the planning.

Vivek Ramaswamy, an entrepreneur, has invested $10 million of his own money in his campaign. Like Mr. DeSantis, Mr. Ramaswamy sells a similar anti-woke sentiment, but he does so with the charm of a natural communicator.

Mr. Trump has welcomed the non-DeSantis entrants to the race. In January, when Nikki Haley, who served as Mr. Trump’s ambassador to the United Nations, called to tell him she planned to run, Mr. Trump did not rant about her disloyalty, as some had expected. He sounded unbothered, telling her to “do what you’ve got to do,” according to two people briefed on their conversation.

And in the days leading up to Mr. Scott’s announcement, Mr. Trump was watching Fox News in his Mar-a-Lago office when he said, “I like him. We’re just going to say nice things about Tim,” according to a person familiar with his private comments.

The conventional wisdom at the beginning of the year was that the field would be relatively small, perhaps as few as five people running. Republican anti-Trump donors were working to thin the herd to prevent a repeat of the divided field that guaranteed Mr. Trump’s victory in 2016. Now, after Mr. DeSantis’s early stumbles, there will likely be as many as 10 candidates competing for attention and vying for the debate stage.

For Mr. DeSantis, the squeeze was apparent on the day he entered the race.

In New Hampshire, Ms. Haley mocked him on Fox News as merely “copying Trump,” down to his mannerisms. “If he’s just going to be an echo of Trump, people will just vote for Trump,” she said.

In Iowa, Mr. Pence sat down with the type of mainstream media outlets that Mr. DeSantis has shunned, including The Des Moines Register. Mr. Pence also met with Bob Vander Plaats, the same evangelical leader Mr. DeSantis had recently brought to Tallahassee for a private meal.

The split screen was a reminder that Mr. DeSantis is being pinched both ideologically and geographically, as the field expands.

Mr. Pence and Mr. Scott have made plain their plans to vie for influential evangelical voters in Iowa. In New Hampshire, both Mr. Christie, who focused his campaign on the state in 2016, and the state’s sitting governor, Chris Sununu, a moderate who has left the door open to a run, threaten to siphon votes from Mr. DeSantis. And in South Carolina, he will be sandwiched between two home-state candidates, the former governor Ms. Haley and Mr. Scott.

Many Republicans who want to defeat Mr. Trump are aghast at the exploding field — along with Mr. DeSantis’s underwhelming performance in recent months. Mr. DeSantis has slipped in the polls and now trails Mr. Trump in all states and by an average of more than 30 percentage points nationally.

“All Republicans have to be hitting Donald Trump,” said Mr. Sununu, who described himself as “50-50” about entering the race. “Any Republican that isn’t hitting Donald Trump hard right now is doing the entire party a disservice because if only one or two people are willing to take a shot at Donald Trump, it looks personal. It looks petty.”

So far, Mr. Christie has gotten the most attention for his direct attacks on Mr. Trump, which he has signaled would be crucial to his candidacy. But he also has delighted in needling Mr. DeSantis at times, an acknowledgment of the Florida governor’s position in the race.

The reluctance to go after Mr. Trump, for many Republicans, feels eerily like a repeat of 2016. Then, Mr. Trump’s rivals left him mostly alone for months, assuming that he would implode or that they were destined to beat him the moment they could narrow the field to a one-on-one matchup, a situation that never transpired.

The two Florida-based candidates in that race, Senator Marco Rubio and Jeb Bush, a former governor, spent millions of dollars strafing each other. Senator Ted Cruz of Texas, who wound up as Mr. Trump’s top rival, gloated privately to donors that he was bear-hugging Mr. Trump while also patiently waiting for the moment to pounce. It never came.

Mr. Trump’s current rivals seem exasperated by their collective inability to crack his foundation: Mr. Trump’s supporters have been trained for years to come to his defense whenever he is under fire.

Mr. Trump has another asymmetrical advantage: Current and potential rivals have sought to avoid criticizing him too harshly so as not to alienate Republicans who still like Mr. Trump and are automatically suspicious of anyone attacking him. By contrast, other 2024 contenders have shown no hesitation in going after Mr. DeSantis.

“His team — maybe him — is excellent at manufacturing the veneer of courage without actually delivering on the real thing,” Mr. Ramaswamy said in an interview last month. “And that can work across TV and even social media,” he added. “But once you poke a little bit, it’s like a little bubble in the air: A little touch, and it’s burst.”

Mr. Ramaswamy, who has criticized Mr. Trump, has aimed most of his fire at Mr. DeSantis. A close friend of Mr. Trump’s son-in-law, Jared Kushner, Mr. Ramaswamy dined with Mr. Trump and Mr. Kushner at the former president’s New Jersey club, Bedminster, in 2021, according to two people familiar with the event.

And while the field grows, there is the matter of the debate stage, where Mr. Trump eviscerated his opponents in the 2016 primary.

The chair of the Republican National Committee, Ronna McDaniel, said earlier this year that she did not expect to need two debate stages as the party required in 2016, with the tiers of candidates determined by polling.

But there could be as many as a dozen declared candidates by August, and many are already racing to collect the 40,000 donors and 1 percent polling threshold the party has indicated will be needed to get onstage. This pool includes longer-shot candidates like Larry Elder, the talk radio host who got walloped in the California recall election.

“Everyone says, ‘We have to keep people from getting in.’” Mr. Sununu said. “That’s the wrong message, the wrong mentality, and that’s not going to work.”

But he acknowledged that consolidation will eventually be needed to defeat Mr. Trump.

“The discipline,” Mr. Sununu added, “is getting out.”